A planar filter is a radio frequency (RF) filtration device having all of its circuitry residing within a relatively thin plane. To achieve this, planar filters are generally implemented using flat transmission line structures such as microstrip and stripline transmission lines. These transmission line structures normally include a relatively thin, flat center conductor separated from a ground plane by a dielectric layer. Planar filters have been of interest in recent years because of their relatively small size, low cost and ease of manufacture.
Planar filters generally include one or more resonator elements. A resonator element is a transmission line configuration that is known to "resonate" at a certain center frequency. In general, a plurality of these resonator elements are arranged to achieve a desired filter response. For example, the resonators can be arranged so that only a predetermined range of frequencies (and harmonics of such) are allowed to pass through the filter from an input port to an output port. This type of filter is known as a "bandpass" filter and the predetermined range of frequencies is known as the pass band of the filter. In another arrangement, the resonators can be configured so that all frequencies are allowed to pass from an input port to an output port except for a predetermined range of frequencies (and harmonics of such). This type of filter is known as a "bandstop" filter and the predetermined range of frequencies is known as the stop band of the filter.
In tunable planar filters, the center or resonant frequency of the filter is altered to alter a characteristic of the outputted RF signal. For example, the range of frequencies (and harmonics of such) passed in a bandpass filter and stopped in a bandstop filter can be altered by altering the resonant frequency of the resonator element(s). To realize tuning, some tunable planar filters pass the RF signal through a ferroelectric material and bias the material with a variable DC voltage source to alter the permittivity of the material. The alteration of the permittivity alters the resonant frequency of the resonator element.
In designing a tunable planar filter, there are a number of important considerations. For example, the tunable planar filter should display very low insertion loss in the pass band of the filter (for bandpass filters) and outside of the stop band (for bandstop filters). The tunable filter should minimize parasitics and other unwanted resonances when the RF signal passes through the tunable filter. The tunable filter should have a high degree of tuning selectivity and sensitivity. The tunable filter should have a compact size for use in components where space is at a premium. The tunable filter should require a modest amount of power to effectuate tuning. Finally, the tunable filter should be robust and reliable in operation.